1. Related Applications
This application is related to U.S. patent application, Ser. No. 726,753, filed on Apr. 25, 1985, now U.S. Pat. No. 4,680,793, and to Japanese Patent Application 60-97962, corresponding to U.S. patent application Ser. No. 861,184, filed May 10, 1985 both by the present inventors.
2. Field of the Invention
This invention relates to a sound multiplex receiver for receiving sound multiplex broadcasts and which is suitable, for instance, for vehicles.
3. Background Art
One example of a conventional receiver of this type is a television sound multiplex receiver as shown in FIG. 1. In FIG. 1, a sound multiplex discriminating circuit 1 discriminates whether a broadcast is a monaural, stereo or bilingual (multi-language) broadcast. The discriminating circuit 1 includes a Q-signal detecting circuit 1a for subjecting a Q-signal of 3.5 f.sub.H (=55.1 kHz) to AM detection and a stereo/multi-language discriminating circuit 1b for discriminating between stereo and multi-language broadcasts. Further in FIG. 3, a logic circuit 2 controls a matrix circuit 3 with the aid of the output of the sound multiplex discriminating circuit 1 and a mode setting signal. A band-pass filter (BPF) 4 transmits a sub-signal and a Q-signal. A subsignal demodulation circuit 5 receives an output of the band-pass filter 4 and feeds the matrix circuit 3 through a low-pass filter 7. Another low-pass filter 6 receives a sound detection signal and directly feeds the matrix circuit 3.
In the receiver thus organized, a sound detecting circuit (not shown) applies the sound detection signal to the band-pass filter 4 and the low-pass filter 6. Of the signal thus applied, a main signal (L+R) or a main sound signal is supplied through the low-pass filter 6 to the matrix circuit 3, while carriers at frequencies of 2 f.sub.H and 3.5 f.sub.H are passed by the band-pass filter 4. The 3.5 f.sub.H carrier is applied to the sound multiplex discriminating circuit 1, while the 2 f.sub.H carrier is applied to the sub-signal demodulation circuit 5. The sub signal demodulation circuit 5 provides a sub-signal (L-R) or a sub-sound signal at its output terminal, which is applied through the low-pass filter 7 to the matrix circuit 3. As a result, the matrix circuit 3 outputs sound signals separated into the right and left channels.
In the sound multiplex discriminating circuit 1, the 3.5 f.sub.H carrier picked up by the band-pass filter 4 is subjected to AM detection by the Q-signal detecting circuit 1a, to obtain a control signal of 922.5 Hz (multi-language) or 982.5 Hz (stereo). According to the control signal thus obtained, the stereo/multi-language discriminating circuit 1b performs a stereo/multi-language broadcast discrimination. The stereo/multi-language discriminating circuit 1b is generally made up of a PLL (phase-locked loop) circuit. In the discriminating circuit 1b, the free run frequency is set between 922.5 Hz and 982.5 Hz, and the content of the control signal is discriminated by detecting the frequency at which the locking (lock-in) is accomplished.
Discrimination of a monaural broadcast and a sound multiplex broadcast can be achieved in different ways, either by detecting the level of the 3.5 f.sub.H carrier, by detecting the level of the detection output of the Q-signal detecting circuit 1a, by detecting the locking of the PLL circuit of the stereo/multi discriminating circuit 1b, or by detecting the level of the sub-signal applied to the sub-signal demodulation circuit 5, or by combinations of the three methods.
A signal discriminating a monaural, stero or multi-language broadcast as described above, and the mode setting signal are utilized to control the matrix circuit 3 which acts to separate the channels of the broadcast according to the selected mode.
It is assumed that the above-described sound multiplex receiver selects the sub-sound in a bilingual broadcast as shown in the timing diagram (b) of FIG. 2, and the field strength is first decreased to a level S.sub.1 and lower and then later restored as shown in the field strength dependence (a) of FIG. 2. In this case, when the externally applied field strength is itself decreased, a monaural reception is effected. However, even after the field strength has been restored, because of the hysteresis or time constant of the sound multiplex discriminating circuit 1, the main monaural sound is continuously outputted until the field strength is restored to a level S.sub.2, and then the sub-sound is outputted again, as shown in the timing diagram (b) of FIG. 2.
This phenomenon occurs repeatedly when the field strength is weak and a flutter interference is caused by an airplane, or when a broadcast wave is received while the receiver is moving in a transmission field including multi-path transmissions. In this case, reproduction of the main sound and reproduction of the sub-sound are frequently switched between the two modes and the resultant sounds are not deemed to be acceptable. Furthermore, when pulse-like noise is mixed with the broadcast transmission, sometimes the Q-signal is erroneously detected. That is, problems arise in the presence of a low signal-to-noise ratio.
The above-described erroneous operations may be eliminated to a certain degree by increasing the time constant of the sound multiplex discriminating circuit. However, the method is disadvantageous in the following points. The characteristic of response to the starting and the ending of a sound multiplex broadcast is lowered. Even if, after the field strength is maintained low for a long period of time thus causing a monaural reproduction, the field strength is restored, it takes a relatively long time for the receiver to operate in the sound multiplex mode.
In U.S. patent application Ser. No. 861,184, corresponding to Japanese Patent Application 60-97962, herein incorporated by reference, the present inventors described a sound multiplex receiver, shown in FIG. 3, built according to a USA or West German standard. FIG. 3 will not be described further except as required. However, it is here pointed out that the receiver of FIG. 3 has drawbacks in that a stero identification circuit 26 and a SAP (separate audio program) identification circuit 28 are both composed simply of comparators. As a result, they are likely to malfunction in the presence of fluctuations of the input field strength.